What is a Yolk Sac?

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  • Written By: Mary McMahon
  • Edited By: O. Wallace
  • Last Modified Date: 08 December 2018
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When people think of yolks, they often think of the nutrient rich parts of eggs which are used to provide nutrition to creatures like birds and reptiles as they develop. However, mammals also develop yolks, in the form of a yolk sac, one of the earliest structures to appear in the course of embryonic development. Pregnant women are often interested in the sac because it is usually the first thing to become visible on ultrasound, while doctors like to use it to gauge embryonic development.

Much like the yolk of a bird's egg, the yolk sac is designed to provide nutrition to the developing embryo. This structure first becomes visible in ultrasounds at about five weeks. As the embryo grows, much of the sac is eventually absorbed into its body, and it gets nutrients from its mother. The vestiges of the sac can be seen in the afterbirth. Women who are interested can ask to see their afterbirth and have the various components explained.


In the very early stages of pregnancy, the body forms what is known as the gestational sac, which envelops the embryo as it develops. When an ultrasound is performed, the sac is usually the most noticeable feature, with the yolk sac being positioned between the embryo and the uterine wall. A doctor may perform measurements of the yolk sac to gather information about the pregnancy and its progress. Irregularities in the size of the sac can indicate that a problem is occurring, and the mother may be at increased risk of miscarriage or complications.

As the embryo is developing and taking nourishment from the yolk sac, which also acts as a crude circulatory system, the placenta between the embryo and the uterine wall is also developing. Eventually, the placenta will be able to take over the duties of supplying nutrients and handling blood circulation. At this point, the contents of the sac are more or less depleted.

When an ultrasound is performed in the early weeks of pregnancy to confirm that a pregnancy is occurring and that conditions appear to be normal, the technician will usually be happy to point out the various structures which can be seen on the screen or in the image. He or she can show the expecting mother where the sac is, and provide an estimate of how far along she is on the basis of the size of the sac and other factors.


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Post 7

It is nice to know what the yolk sac is because when they told me that's what it was I kind of looked at them funny. I was told it was a good thing and that it was normal and it looked good.

Post 6

I was told that my yolk sac was too large and I had some blood in my uterus. I searched the internet and found all of the information about these two things to be quite disheartening. No good news to be found.

Thankfully, I now have a two month old baby boy who is completely healthy. I did have other difficulties during the pregnancy, however everything turned out good, so you are not doomed.

Post 5

I too had a pregnancy miscarriage due to the abnormality in the yolk sac (6.7 mm), I am 36 yrs old. My doctor says age is the main factor for this miscarriage. Any advice on how to avoid this in my next pregnanacy?

Post 4

I'm seven weeks pregnant. When I got done with the ultrasound, my doctor said that the hear is not yet beating and even the yolk sac is measuring 6.7 mm. So she said the chances are not good. Can any of you advise me, please, because I'm very much worried.

Post 3

I hope I don't sound too idiotic, but is the yolk sac independent of what I eat? How does the yolk sac gets it's nutrients to give to the baby? All I've read so far makes it sound independent.

Post 2

Very nice article -- what I'd like to know is, does the yolk sack have an impact on fertilization and pregnancy?

For instance, can a woman have an abnormal yolk sack that could cause them to have problems getting pregnant?

And what happens to the yolk sack during the development of pregnancy?

I assume it enlarges, but what else happens?

Post 1

That's so weird -- I had no idea that women had yolk sacs. The things you learn online...

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